A permanent-magnet field rotor is often used in synchronous motors or the like. Traditionally, the permanent-magnet field rotor includes a plurality of permanent magnets arranged in a radial fashion and yokes disposed between each two successive permanent magnets and made of a number of stacked sheet elements. Each of the permanent magnets has a generally rectangular cross-section which extends in the radial direction, and each magnet has circumferentially opposed side faces which are magnetized into opposing poles N and S, respectively. Each yoke has a generally sectoral cross-section and has circumferentially opposed side faces, each of which is in close contact with the same pole side face of each of the adjacent permanent magnets. The outer circumferential faces of the yokes provide alternately different poles. Each two successive yokes disposed at opposite sides of each of the permanent magnets are separated from each other to prevent the yokes from forming a magnetic short-circuit. Each yoke is provided at either corner between the circumferential face and the side faces thereof with protrusions each overlapping the outer side face of each of the adjacent permanent magnets, to prevent them from moving outwardly in the radial direction.
Each yoke is made as a one-piece solid body or as a stack of a number of sheet elements.
In the case of manufacturing the permanent-magnet field rotor including yokes each made of a number of stacked sheet elements, the yokes were heretofore formed by stacking the sheet elements, each of which was formed by press punching, and having a generally sectoral shape as a final shape. The yokes and the permanent magnets were then firmly secured to each other in such a manner that the permanent magnets were disposed between each two successive yokes.
In this case, however, since it was necessary to form a plurality of stacks of sectoral sheet elements in accordance with the number of yokes, a long operating time for forming the plurality of stacks was required. Furthermore, the sectoral sheet elements in each of the stacks were apt to slip and move apart from each other, and thus it occurred that they became oriented in different directions. Consequently, the insertion of the permanent magnets into the space between each two successive stacks or yokes was difficult and troublesome. Therefore, it was difficult to improve the productivity of the permanent-magnet field rotor in the conventional method of manufacturing the same.
In order to facilitate the insertion of the permanent magnets between each two successive yokes, a cylindrical jig surrounding the stacks has been used to limit the position of the sectoral sheet elements. In this case, although the regulation of orientation of the sheet elements was improved along the stacking direction, the operation for stacking the sectoral sheet elements within a narrow space inside the cylindrical jig became more difficult. Therefore, it was also difficult to improve the productivity of the permanent-magnet field rotor.